It is known, in providing protective circuitry for an alternating current electric motor, to provide means responsive to the current traversing a winding thereof and operating a switch or other device for open-circuiting the motor upon the development of a current whose actual value exceeds a predetermined nominal value assigned to the circuit by the dimensioning of the circuit elements thereof.
Such an overload-protective circuit can include a current transformer whose primary winding is connected in series with the motor winding to be protected and the line-current source. The transformer also has a secondary winding which forms part of an actual-value current-monitoring circuit or a voltage-supply network for the circuit.
Such a circuit arrangement has been described, for example, in German published application (Auslegeschrift) DT-AS No. 15,88,896, the circuit of which responds to both fault currents and currents exceeding a nominal value, i.e. a current overload. In this circuit, two current transformers are provided whose primary windings are each connected to the terminals of the motor, i.e. to the respective phase terminals of a polyphase motor or to the neutral point and one or more phase terminals.
In the secondary winding of the first current transformer, a secondary current is induced which is a function of the sum of the primary current in the phase windings and at the neutral terminal.
The secondary of the second current transformer comprises three secondary windings corresponding to the phase windings of the motor and connected in a star configuration. The current induced in these secondary windings is converted into a voltage so that these secondary windings serve in part as a voltage supply network. The secondary windings of the two current transformers are separate from one another.
As noted previously, such a circuit must be designed specifically for each nominal value of the maximum current for each electric motor with which the circuit is to be associated. In other words, one cannot use one and the same circuit for motors having different nominal current values. This poses a problem since the protective circuitry must be redesigned for each nominal current of motors with which they may be used and a large number of different protective circuits with different nominal values may have to be stocked if service is required for a variety of different capacity motors.
U.S. Pat. No. 3,544,846 describes an overload circuit in which two current transformers are provided in cascade and the effective number of turns of the primary winding of the second transformer can be varied. This, however, only allows compensation for the relay which is to be operated by the circuit to disconnect the motor and even this circuit has not been found to be effective for use with a large number of different capacity motors with different nominal currents.